In a recent article in photo technique Magazine (Nov/ Dec 2012), Dan Burkholder provided a helpful overview of focus or z-stacking. The technique is most frequently applied in macrophotography. In this article I share some special tips and hints that may not be apparent and may even appear counterintuitive to anyone interested in trying this technique.
Lighting should be consistent both in directionality as well as intensity in all frames of the stack. Accordingly, light sources should not be mounted on the camera or lens, which is moved in the process of z-stacking. The changed position of the lights can alter the shadows and highlights on the object.
I mount the flash heads of my Canon MT-24EX on Wimberley Plamps. I modified one of the links in the arms by attaching flash mount shoes with a screw and setting it in epoxy resin (superglue does not work).
To ensure consistent exposure of all frames, manual exposure is advised, both with ambient light as well as with flash. Highlights have a tendency to blow out in the z-stacking process, so the familiar “expose to the right” maxim of digital imaging should not be pushed.
Rather, expose so that the histogram ends about one f-stop to the left of the white point. When capturing RAW files, overexposed stacks can be batch-processed in Photoshop with an exposure adjustment action.
The intention of z-stacking is to achieve extended focus. It may sound reasonable to use the greatest f-stop to generate the greatest depth of field for each of the frames. However, it is a better approach to use the lens at its sharpest and let the z-stacking program do the work of generating the depth. Most lenses are sharpest around 1 to 2 f-stops down from fully open, which is the ideal f-stop for z-stacking. Also remember, that diffraction introduces image blur, so maximum advantageous f-stop (fmax) is 32/M+1. At 1:1 = f/16, at 5:1 = f/5.3. For work on stereomicroscopes it is best to leave the f-stop (if available) all open.